An image recording apparatus using a plurality of optical beams, wherein each of the optical beams has a different wavelength is known. In such an apparatus, density signals which express the image of a document are converted into optical signals with a gas or semiconductor laser to record the image on a photosensitive material. Especially well known is an apparatus for forming a color image with trichromatic beams.
In this apparatus, a plurality of optical beams having desired densities are emitted, as shown in FIG. 1, and are composed into one optical axis by using a dichroic mirror M.sub.0 and half mirrors M.sub.1 and M.sub.2 so that the optical beams may expose a photosensitive material at a substantially common point. In another known apparatus the photosensitive material is irradiated with optical beams which are perpendicular or parallel to the scanning line In this apparatus a predetermined position on the photosensitive material can be irradiated with the optical beams of the desired densities by controlling devices which shift the emission timings of the individual beams In the case of the former apparatus shown in FIG. 1 the point of incidence upon a polygon mirror PM for the scanning and exposing operations can b.RTM.focused by integrating the optical axes so that the polygon mirror and its drive motor can have their sizes reduced. Despite this advantage, however, the number of parts in the optical system is increased, thereby making the optical system complex, and making the adjustment for composing the optical axes delicate so that the time period for the adjustment necessarily becomes long Another disadvantage is that the optical beams are attenuated by the mirrors, thereby decreasing the amount of exposure at the focal point. In the case of the latter apparatus, the irradiated positions on the polygon mirror are different from each other, thereby causing a problem in that the polygon mirror and its drive motor need to be of a relatively large size in order to cover the required scanning range on the photosensitive material.